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Understanding X-RAID and capacity
I'm thinking of upgrading from an almost full ReadyNAS Duo (with two 2GB disks) to an RN10400. I've got a spare 2GB disk and plan to buy an additional 4TB disk. Given these three 2GB disks and th...
geojay wrote:
I don't understand your logic I'm afraid. I understand that the 4TB drive will only function as a 2TB drive in this configuration so let's treat it as a 2TB drive and then we have four 2TB drives or 8TB total. I'm assuming everything is redundant so does that not give us 4TB of effective capacity?
Nope.
With this disk configuration XRAID uses RAID-5. Here's a (somewhat simplified) picture of how RAID-5 redundancy works. It's a math trick.
Imagine 3 blocks of data - A, B, C. These blocks are stored on three of the disks.
Now compute P=A+B+C, and store that in the corresponding slot of the fourth disk.
If any disk fails, you can compute the missing block from the corresponding blocks on the other three disks.
A = P-B-C
B = P-A-C
C = P-A-B
P = A+B+C
So you are protected from a single disk failure. The overhead is 25% in this case, not 50%.
XRAID is a more sophisticated when you have mixed disk sizes. But even with mixed disk sizes, capacity = (sum of all drives)-largest
OK, 'math trick', I'll leave it there... :)
So is to say that if I eventually populate it with four 4TB drives (I'm not actually sure what the maximum drive size is after looking at the specs, it just says 16TB maximum, I assume that's actual storage distributed across the four bays?!) then that'll give me 12TB of effective storage?
Thanks for your assistance!
geojay wrote:
So is to say that if I eventually populate it with four 4TB drives ... then that'll give me 12TB of effective storage?
Yes.
You can replace a drive with one of identical size, or with one that is at least as large as the biggest one current in the array. You can't install a smaller size.
If a slot is empty, add a drive that is at least as large as the biggest one in the array.
Also (in case you are not aware), drive manufacturers use TB units (1000*1000*1000*1000 bytes); Windows and ReadyNAS use TiB units (1024*1024*1024*1024 bytes). 1 TB = .9 TiB. So a 12 TB volume will be reported as 10.9 TiB. Unfortunately both Windows and ReadyNAS use the "TB" label for TiB, which makes this unit mess even more confusing.
geojay wrote:
...I'm not actually sure what the maximum drive size is after looking at the specs, it just says 16TB maximum...
There is no known limit in the firmware. However, if you are using xraid, then 4 TB drives are a good ceiling for the RN104. Bigger drives will work (I have a 6 TB and an 8 TB in my RN102), but RAID sync times get long. RAID sync isn't a problem for me, since I am using jbod in the RN102 (it is a backup of my main NAS, so I opted for capacity instead of failure protection).
FWIW, if your budget will stretch to an RN204 then I'd suggest going for that instead of the RN104. It is quite a bit faster.
FWIW, if your budget will stretch to an RN204 then I'd suggest going for that instead of the RN104. It is quite a bit faster.Ah, that's food for thought as there's not a massive difference in price...
Can you point me at any meaningful comparisons of the two devices? I'm not managing to find anything that lets me compare one with the other...
Normally I use smallnetbuilder.com reviews for this - but at the moment I get an html error on those sites.
The RN100 series has a single-core 1.2 ghz ARM processor with 512 mbyte of ram, the RN200 has a dual-core 1.4 ghz processor with 2 gigabyte of ram.
I have an RN102 and an RN202, but neither are set up for raid. The RN202 is definitely faster, and if the price is close it's worth getting instead.
The RN214 has a quadcore processor, and if you want on-the-fly video transcoding it is the one to buy. However, there's a bigger price jump there, and I don't think you'll see much difference in file transfer speeds.
RN200 has a dual-core 1.4 ghz processor with 2 byte of ram.
2 gigabytes of RAM.
The core is also a newer generation of the architecture using the Annapurna Alpine chipset, assume this will make a small difference.
Search for "readynas rn204" and you should find a few reviews, but I've not seen any comparing the RN104 with the RN204 (I have seen RN204 vs RN214), e.g. RN104 review, RN202 review. As Stephen mentions, the RN204 is a bit quicker and also a two year newer design, therefore it would be my choice.
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